A die-sinking electrical discharge machine performs electrical discharge machining on a machining object by producing a pulse discharge in a machining gap that is formed by disposing a machining electrode and the machining object opposite to each other. In a die-sinking electrical discharge machine, a power source controller controls a machining power source used for applying a machining voltage to the machining gap and controls the discharge pulse width (discharge duration) and the quiescent period of the pulse discharge produced in the machining gap.
Thus, a power source controller of a die-sinking electrical discharge machine is required to have the capability of maintaining the machining state at an optimum state. Hence, conventionally, various technologies have been disclosed for determining the electrical discharge state at the time of machining discharge (for example, Patent Literature 1, Patent Literature 2, etc.)
For example, in Patent Literature 1, a technology is disclosed for determining the electrical discharge state at the time of machining discharge by making use of a magnitude relation between a high-frequency component, which is superimposed on the discharge voltage at the time of machining discharge, and a reference value (see FIGS. 7 and 8 in Patent Literature 1). In addition, for the case when a discharge pulse produced in the machining gap is determined to be an abnormal discharge pulse on the basis of the magnitude relation between the high-frequency component and the reference value, a technology is disclosed for controlling the machining conditions in such a way that the electrical discharge state and the machining efficiency improves with the control of the quiescent period (see FIGS. 30 and 31 in Patent Literature 1).
In Patent Literature 2, a technology is disclosed for determining the electrical discharge state from a discharge voltage level at the time of electrical discharge machining that is detected using a reading window (see FIG. 1, FIG. 2 in Patent Literature 2).    Patent Literature 1: Japanese Patent Application Laid-open No. H5-293714 (FIG. 7, FIG. 8, FIG. 30, FIG. 31)    Patent Literature 2: Japanese Patent Application Laid-open No. S61-159326 (FIG. 1, FIG. 2)